1. Field of the Invention
The invention generally relates to processes and equipment for dewatering fine coal.
2. Description of the Prior Art
Most coals are cleaned at relatively coarse sizes typically in the range of two inches and 48 mesh. Finer coals are often discarded because of the high costs of processing. Although the amount of the fines discarded is relatively small as compared to the coarse particles that are cleaned, it represents a significant loss of valuable resources and creates environmental problems. It is estimated that there are approximately 500-2,000 million tons of the fine coals in abandoned refuse ponds and 500-800 million tons of fines in active refuse ponds in the U.S. Despite the technological advancement made in recent years, the U.S. coal industry is still discarding 30-50 million tons of the fine coal to refuse ponds.
There are two reasons for the high costs of processing fine coals. One is the low efficiency of cleaning and the other is associated with the high cost of dewatering. The first problem has been resolved to a large extent by the advent of advance coal cleaning technologies such as microbubble column floatation and selective agglomeration. These water-based processes are capable of recovering the fine coal from finely dispersed ash- and SO.sub.2 -forming minerals; however, it is difficult to remove the free water adhering to the surfaces of the fine coal particles. The finer the particle, the larger the surface area and, hence, the more difficult it becomes to dewater the clean coal product. Typically, 100 mesh.times.0 flotation products contain 30-40% moisture after a mechanical dewatering process such as vacuum filtration, causing not only a loss of heating values, but also problems with handling and transportation. Some consider that cleaning fine coal replaces one type of inert substance (i.e., ash-forming minerals) by another (i.e., water), offering no financial incentives for coal companies to clean fine coals. Thermal drying can remove the moisture, but it is costly and usually requires cumbersome permitting processes. The costs of thermal drying is estimated in the range of $2-25 per ton of coal, which are substantially higher than those for mechanical dewatering processes.
Many investigators suggested methods of improving the efficiency of mechanically removing water from bituminous coal fines. These include polymer addition, surfactant addition and use of electrical or acoustic energy to aid in the dewatering process. Some of these methods showed improvements in dewatering rate, but not necessarily in reducing the final moisture content. The use of high pressure filters vastly improved the kinetics and reduced the final moisture contents beyond what can be achieved with conventional vacuum filters; however, the final moisture contents are still far above the levels that can be achieved by thermal drying. Furthermore, the high-pressure filters suffer from high capital and maintenance costs.
The most commonly used mechanical dewatering devices are vacuum filters. With this technique, the finer particles fill the voids between coarser particles in the filter cake, significantly increasing the pressure drop. Various flocculating agents, such as organopolysiloxanes as disclosed in U.S. Pat. Nos. 4,290,896 and 4,290,897, are designed to minimize the blockage by flocculating the particles and thereby increase the filtration rate. Various surfactants have also been used as dewatering aids, the role of which is to increase the filtration rate rather than reduce the final moisture content.
There is an entirely different kind of dewatering problem than discussed above facing the coal industry. The low-rank coals mined in the western U.S. contain 30-35% moisture as they are formed underground. The water in these coals are referred to as inherent moisture as it constitutes an integral part of the coal structure, and is distinguished from the free moisture adhering to the surface of higher-rank coals. The only way to remove the inherent moisture is to subject the coal to high pressure and/or temperature, which is substantially more costly than removing the free moisture from the higher-rank coals such as bituminous coals. There are many different methods of upgrading low-rank coals by removing the inherent moisture.
Some of the low-rank coal beneficiation techniques describe methods of removing water after removing the inherent moisture. For example, U.S. Pat. No. 4,185,395 to Nakako et al. discloses a method in which brown coal mixed with hydrocarbon oil is heated to 100.degree.-130.degree. C. and then passed through a gas-liquid separation process to separate the slurry into a stream containing the hydrocarbon vapor and a dehydrated slurry. The hydrocarbon oil is recycled in the process. The Nakako et al. process suffers from the drawback that it is a thermal drying process which is energy intensive.
The U.S. Pat. No. 3,992,784 to Verschuur et al. also discloses a method of heating an aqueous slurry of brown coal to 150.degree. C. in the presence of hydrocarbon oils. In the example experiments n-C.sub.12 hydrocarbon oils have been used to obtain products containing moisture in the range of 31 to 54%.